<p>Nanotwinned metallic thin films offer a promising route to simultaneously achieve high strength and excellent electrical conductivity, making them attractive for electronic applications and advanced packaging. However, the microstructural characteristics and properties of nanotwinned films in alloy systems remain insufficiently explored. In this study, Ag-Cu alloy thin films with high-density nanotwinned structures were successfully fabricated via magnetron cosputtering. Microstructure and underlying mechanisms were investigated using x-ray diffraction (XRD), transmission electron microscopy (TEM), electron backscatter diffraction (EBSD), focused ion beam (FIB), and atomic force microscopy (AFM). The results reveal that Ag-Cu alloying leads to a reduction in stacking fault energy, which in turn facilitates the formation of high-density nanotwins with reduced twin spacing and refined grains. The Ag-Cu alloy nanotwinned films exhibit a strong (111) preferred orientation, reaching up to 98.1%. Moreover, the nanotwinned structure in the Ag-Cu alloy films demonstrate notable thermal stability after annealing at 600°C. The nanotwinned structures remain, which is a distinct contrast to the rapid grain growth typically observed in pure metal nanotwinned films. The experimental results are also discussed in relation to the underlying mechanism. These insights offer a framework for the design of alloy-based nanotwinned films in electronic and structural applications.</p>